Article pubs.acs.org/IECR
Solid-State Foaming of Cyclic Olefin Copolymer by Carbon Dioxide Zhenhua Chen,†,‡ Changchun Zeng,*,†,‡ Zhen Yao,*,∥ and Kun Cao§,∥ †
High Performance Materials Institute, Florida State University, Tallahassee, Florida 32310, United States Department of Industrial and Manufacturing Engineering, FAMU-FSU College of Engineering, Florida State University, 2525 Pottsdamer Street, Tallahassee, Florida 32310, United States § State Key Laboratory of Chemical Engineering, Zhejiang University, Hangzhou 310017, China ∥ Institute of Polymerization and Polymer Engineering, Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310017, China ‡
ABSTRACT: Solid-state foaming of cyclic olefin copolymer (COC, Topas 6017) using carbon dioxide (CO2) was investigated. Before the foaming experiment, the solubility of CO2 in the polymer at various saturation pressure (5−10 MPa) and saturation temperature (40 °C) was measured using the ex-situ gravimetric method. The effects of the saturation pressure, foaming temperature, and foaming time on the foam structure were systematically investigated. Ultramicrocelluar foams with a cell size of 0.5 μm and cell density over 1012 cells/cm3 were successfully prepared. Moreover, it was observed that the system, which was strongly plasticized by CO2, would transition from foaming to crazing under certain conditions. The phenomenon was examined in detail and understood by a proposed mechanism that accounts for both homogeneous nucleation and stress fields induced crazing.
1. INTRODUCTION Microcellular foams were initially developed by Martini and coworkers at MIT1 using inert gases (carbon dioxide and/or nitrogen) as the physical-blowing agents. Generally, these foam materials are characterized by a cell density higher than 109 cells/cm3 and a cell size of 10 μm or less. They have attracted a great deal of interest in the past few decades due to their unique capability of offering a new range of insulating and mechanical properties with concurrent reduction in materials weight and cost.2,3 Microcellular polymers have been shown to possess high impact strength,4−6 high toughness,7 high stiffness-toweight ratio,4,5 high thermal stability,8 and low thermal conductivity9 when compared to their unfoamed counterparts. As a result, they have been increasingly used in a variety of applications for food packaging, insulation, filtration membrane, sports equipment, automobiles, aircraft parts, etc. Several techniques have been developed to prepare microcellular foams by physical foaming using gases in their supercritical or subcritical states.10−13 These techniques are based on the following common principles: (1) dissolution of gas in a polymer to form a polymer−gas solution; (2) quenching of the polymer−gas solution into a supersaturated state by either reducing pressure (pressure quenching method) or increasing temperature (temperature jumping method); (3) nucleation and growth of gas bubbles till the thermodynamic and/or mass transport driving forces vanish. Both pressure quenching and temperature jump methods take advantage of the plasticization effect of the dissolved gas, which depresses the intrinsic glass transition temperature (Tg) of the polymer. If the plasticizing effect and Tg depression are prominent, foaming may take place at temperatures below the normal glass transition temperature of the polymer, leading to a solid-state foaming process.14 During the process, the plasticization effect also causes a significant reduction in the Young’s modulus and © XXXX American Chemical Society
an increase in both the effective yield strain and the elongation at the break of the polymers, which are beneficial for bubble nucleation and growth. The solid-state foaming process has been used to prepare microcellular foams from a number of amorphous and semicrystalline polymers, such as poly(vinyl chloride),15 polystyrene,16 polycarbonate,14 poly(acrylonitrile− butadiene−styrene),17 poly(methyl methacrylate),4 poly(ethylene terephthalate),18 polyetherimide,19,20 poly(ether sulfone),10,19,21 and poly(aryl ether ketone),22 etc. One of the key processes to determine the cellular structure is the nucleation and growth of gas bubbles. The resulting structure depends on a balance between several mechanisms, such as the gas solubility and plasticization effect, gas diffusion, and the momentum and thermal transfer in the polymer, which depend inherently on the polymer and process conditions.23 Cell nucleation phenomena in microcellular foaming have been primarily studied by applying classical nucleation theory.24−26 A different mechanism was proposed by Holl et al.27 in solid-state foaming, in which cell nucleation is caused by a triaxial tensile failure mechanism which is similar to the explosive decompression failure observed in elastomers used under high-pressure environments.28 Cycloolefine copolymers (COCs) are a family of amorphous copolymers of ethylene (E) and norbornene (NB) with varying E/NB ratios, which largely determines their mechanical and thermomechanical properties. For example, a higher norbornene fraction in the COC would result in a polymer with a higher glass transition temperature (Tg). COCs are a relatively new thermoplastic with many excellent properties such as good Received: March 14, 2013 Revised: June 8, 2013 Accepted: June 13, 2013
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a function of time. Sample transfer was completed as quickly as possible (∼40 s after opening the relief valve) to minimize gas loss before measurement. 2.3. Solid-State Foaming. COC sample was saturated for 28 h with CO2 under designated pressure and temperature using the same apparatus for gas solubility measurements. From the solubility study it was determined that under the chosen saturation time equilibrium was reached in all experiments. Following rapid release of the pressure, the sample was removed from the pressure vessel and quickly transferred ( Tg). However, after the pressure was released and sample taken out of the pressure vessel, no foaming was observed. Instead, a massive amount of macroscopic lines were observed in the sample (insert of Figure 13). The lines were cracks that can only result from growth of crazes. Under the saturation condition used, the COC matrix plasticization is extremely severe and hence the severe reduction of matrix strength. In addition, an arguably enormous stress field is developed after the sample was taken out of the pressure vessel because of the extremely high gas solubility and enhanced diffusivity. The experimental conditions used herein thus exemplify the driving forces for crazing to an extreme extent. M
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Scheme 2. (a) Bubble Formation from Homogeneous Nucleation under Low Saturation Pressure; (b) Crazing Formation from the Cooperative Effects of Triaxial Tension and Tensile Stress Field under High Saturation Pressure; (c) Simultaneous Formation of Bubble and Crazing under Medium Saturation Pressure
described well by the dual-sorption model. Systematic investigation of the effects of the process parameters, e.g., foaming temperature, foaming time, and saturation pressure, was conducted to tailor microcellular foam structures. The resulting foam structures which had average cell diameter in the range of 0.3−7.5 μm and cell density on the order of 109−1012 cells/cm3 can be controlled by manipulating processing conditions. An ultramicrocelluar foam with a cell size of ∼0.3 μm and cell density of ∼1012 cells/cm3 can be obtained at saturation pressures of 6 and 7 MPa. Solid-state foaming of COC was found to be dictated by two competing processes, bubble nucleation vs crazing. The crazing process was analyzed in detail based on the stress field development. The triaxial tensile stress field in the gas− polymer mixture resulted from the volumetric swelling or dilation strain giving rise to nucleation of voids, and the pseudotensile stress field from diffusion resulted in a gas concentration gradient that facilitates growth of the crazes and cracks. As a result of the strong COC−CO2 interaction and accompanied depression of the glass transition temperature and reduction of the matrix rigidity, crazing becomes more prominent with increasing saturation pressure and eventually completely dominates. The significant plasticization of the polymer sample at high saturation pressure facilitated the crazing process at temperatures below the effective glass transition temperature, which is the minimum temperature required for foaming.
That under these conditions the nucleation for crazing completely dominates and foaming has been completely suppressed in the sample provides direct experimental support for the discussed mechanisms. 3.3.3. Coexistence of the Foaming and Crazing Process under Medium Pressure. To summarize sections 3.3.1 and 3.3.2, both foaming and crazing are possible in solid-state foaming of COC. Foaming is favored when the saturation pressure is low and suppressed when the saturation pressure is high. Crazing, on the other hand, has an opposite pressure dependency. It is conceivable that in the intermediate-pressure regimes both the foaming and the crazing processes may be possible. On one hand, voids and crazes may still form because of the stress fields. On the other hand, the less severe stress fields lead to fewer and discrete crazes, whose growth is slow with limited consumption of gas. Thus, bubble nucleation and growth are still possible when the effective Tg is reached, which would compete for the available gas. Furthermore, some of the voids resulting from the triaxial tension may not be able to grow into crazes before the effective Tg is attained. As discussed by Holl et al.,27 they would instead serve as nuclei for bubble growth. These three different processes proceeding under different saturation pressure are schematically illustrated in Scheme 2.
4. CONCLUSIONS In this study, we investigated the solid-state batch foaming of a cycloolefin copolymer (COC) using carbon dioxide. The gas sorption properties that are critical for foaming were characterized first. It was found that gas sorption can be N
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AUTHOR INFORMATION
Corresponding Author
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[email protected] (C.Z.);
[email protected] (Z.Y.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Funding from the Center of Excellence in Advanced Materials by the State of Florida is acknowledged. REFERENCES
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